Having access to the Space does not mean that the user will automatically be able to access everything in it. You have to either make the content visible to everyone in the space or invite specific users to be able to view/edit it. This article outlines how to do it for the Salesforce Org models and standard Reference models.

TIP If the "Invite to Org model" button does not appear in the "Information" tab, then you do not have sufficient permissions to invite other users to this Org model. You need to be the Org model Owner or have been granted "Manager" rights to invite others and manage permissions.

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Every user in the Space has view access to any connected Org Model or Reference model. To change that, select your model, then choose the "Invite to Org model" tab in the right panel and toggle off that setting.

If you would rather limit access to a particular model to a selected few, you can invite them individually using the "Invite to Org model" button on the right side panel. 

Any member of a Space can be invited as a viewer to the Org or Ref Model. In order to grant someone edit or manage rights, however, they already need to have an editor license in the space. 

If a user has edit rights on the Space, they will have the option to change them to an Org model editor or manager in the pick-list next to their name. Otherwise, it is greyed out as "View only".
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The total number of "Space Viewers" is shown at the bottom of the right "Invite to Org model" panel.

Once you have manage or edit rights, you can add and edit documentation and content in the Org Model.